1. Field of the Invention
The present invention relates to a method for depositing a second film on a second face of a substrate by the dipping method of liquid phase epitaxy (LPE) and more particularly pertains to the fabrication of a multiple layer composite wherein two films of materials which are different from each other are deposited on opposite faces of a substrate by the dipping method of LPE.
2. Description of the Prior Art
The epitaxial deposition of films of different materials on the opposite faces of a wafer substrate can be accomplished by any method of epitaxial deposition which affects only one face of the wafer at a time. During chemical vapor deposition (CVD), for example, the wafer rests on a plate with only one face exposed. A film is epitaxially deposited on the exposed face while the opposite face is protected from deposition by the plate. However, CVD epitaxy techniques are not the ones of choice for some applications. It has not been demonstrated that CVD techniques can be controlled to grow complex compositions having precisely determined amounts of substituents or dopants incorporated therein. Such control is important in applications such as, for example, magnetostatic surface wave propagation on ferrimagnetic, or ferrite, films epitaxially deposited on nonmagnetic substrates. Materials are termed "ferrimagnetic" herein in accordance with the definition of ferrimagnetism given in The Encyclopedia Brittanica.
As in CVD, only one face of a wafer is exposed for deposition in the tipping method of LPE. In the latter method, the molten flux is contained in a boat having an elevated, and thus shallow, end. The wafer rests on the bottom of the boat at the elevated end thereof. The bottom of the boat protects the adjacent face of the wafer from contact with the flux and thus, protects that face from deposition thereon.
The tipping method of LPE is known to have certain disadvantages. For one, the method is not isothermal. Isothermal or constant temperature deposition is preferred since uniformity in the composition of the deposited film is usually desired. Another disadvantage of the tipping method of LPE results from the fact that the volume of melted flux involved is usually relatively small. Thus, as the deposition proceeds, the composition of the involved melt changes. As with temperature variations this factor leads to nonuniformity in the composition of the deposited film.
The dipping method of LPE is the preferred method for producing high-quality deposited films. In the case of ferrimagnetic garnet films on nonmagnetic garnet substrates, for example, the films are regarded as having high quality when they have a uniform composition over substantially their entire area which may be relatively large. Low lossiness (or linewidth) for magnetostatic surface wave propagation is an important aspect of the quality of such films. Another advantage of the dipping method of LPE is that it may be used to grow complex film compositions having precisely controlled amounts of substituents or dopants therein by, for example, controlling the particular temperature at which deposition occurs and by stirring the flux to maintain its composition uniform.
When depositing a film on a single wafer by the dipping method of LPE, the wafer is ordinarily and preferably totally immersed in the molten flux. Therefore deposition can and does occur on each of the two faces of the wafer simultaneously.
Ways are known, however, to use the dipping method of LPE to grow a film on one side of a wafer only. Firstly, the surface of the wafer on which a film is to be deposited may be brought into contact with the surface of the flux bath while the opposite surface of the wafer is held out of such contact. Secondly, a wafer may be inserted into a holder together with a metal shield, usually of platinum. The shield is held in sealing contact with one surface of the wafer. The holder and shielded wafer may then be totally immersed in the flux. Neither one of these two approaches is satisfactory when uniformity in the deposited films is desired due to the high thermal gradients which are present at the surface of a molten flux bath or which are introduced deeper in the bath by the presence of thermally conducting metal shield. In addition, as will be seen below, these two approaches have a lower production rate than that of the subject invention.
In Bongianni, U.S. Pat. No. 3,864,647, there is disclosed a magnetostatic surface wave delay line. The preferred embodiment in Bongianni is a layered structure having two substantially identical layers of magnetic-wave-active material disposed on opposite sides of a magnetic-wave-inactive substrate. The layered structure is preferably fabricated by immersing the substrate in an LPE system in which both magnetic-wave-active layers grow simultaneously on the opposite sides of the substrate.
In order to operate the magnetostatic surface wave delay line disclosed in the above-referenced patent to Bongianni as a linearly dispersive or non-dispersive delay line, it is desired that the two layers of magnetic-wave-active material have different magnetic characteristics. Since these two layers have substantially identical compositions, the differing magnetic characteristics are obtained by placing the layered structure in a bias magnetic field having a substantial non-zero gradient substantially perpendicular to the deposited layers. A three-magnet configuration is disclosed for producing this special bias magnetic field.
Special magnet structures having a field with a gradient controlled to produce desired characteristics in a delay line are difficult to design and fabricate. It is preferable to use layered structures which can be operated as linearly dispersive or non-dispersive delay lines in a uniform bias magnetic field. This suggests that the individual magnetic-wave-active films or layers must be different from each other in some way.
Prior art techniques for fabricating such uniform bias magnetic field delay lines are discussed in Ganguly et al, Journal of Applied Physics, Vol. 45, No. 10, October 1974, pp. 4665-4667, "Magnetostatic Wave Propagation in Double Layers of Magnetically Anisotropic Slabs", and in Tsutsumi et al, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-24, No. 9, September 1976, pp. 591-597, "Effect of the Magnetic Perturbation on Magnetostatic Surface-Wave Propagation". These references are more fully discussed in concurrently filed U.S. patent application, Ser. No. 831,040 entitled "Improved Multiple Magnetic Layer Composite for Magnetostatic Surface Wave Propagation" filed by Wayne L. Bongianni on Sept. 6, 1977 and assigned to the assignee of the present application. A notice of Allowance dated Oct. 18, 1978, has been received for this concurrently filed application. The application of Bongianni describes a composite which may be fabricated by the method of the present application.
In Ganguly et al, physically distinct slabs of substantially identical ferrimagnetic materials are given a different crystallographic orientation relative to the bias magnetic field. In Tsutsumi et al, two physically distinct slabs of ferrite having different compositions are used. Neither Ganguly et al nor Tsutsumi et al discloses a monolithic multiple layer composite wherein two films of different materials and, consequently, different magnetic characteristics are epitaxially deposited on opposite sides of the same magnetic-wave-inactive substrate.
A layered composite comprising a substrate having two films of magnetic-wave-active material having magnetic characteristics different from each other, the two films being deposited on the opposite faces of the substrate, may be realized, for example, by causing the compositions or crystal structures of the two films to be different.
As is well known, the magnetic characteristics of a magnetic-wave-active material such as, for example, a ferrimagnetic garnet can be altered by substituting certain metallic ions for others in the garnet. The magnetic characteristics of yttrium iron garnet (YIG) are known to change when ions of other metals are introduced into the crystal lattice as substituents for the yttrium or iron. In the dipping method of LPE, the extent to which some substituents or dopants such as, for example, lead are incorporated into a deposited film can be controlled by varying the temperature of the molten flux. See, for example, copending U.S. Pat. Appln. by Glass et al, Ser. No. 931,713 (a continuation of application Ser. No. 792,159, filed Apr. 29, 1977, and now abandoned which, in turn, was a continuation of parent application, Ser. No. 687,428, filed May 17, 1976, and now abandoned), entitled "Minimization of the Ferromagnetic Resonance Linewidth in Yttrium Iron Garnet Films", filed Aug. 7, 1978 and assigned to the assignee of this application.
For other desired substituents such as gallium or lanthanum, for example, controlling the concentrations of salts of these metals introduced into the molten flux varies the extent of incorporation of the substituents into the lattice structure of the deposited film. See, for example, H. L. Glass, J. H. W. Liaw, and M. T. Elliot "Temperature Stabilization of Ferrimagnetic Resonance Field in Epitaxial YIG by Ga, La Substitution", Mat. Res. Bull. Vol. 12, No. 7, July 1977, P. 735, which discloses that combined substitution of gallium and lanthanum in YIG can be used to shift the field at which resonance occurs in YIG by an amount which is equivalent to a change in the resonance frequency of about one gigahertz for a given fixed bias field.
3. Prior Art Statement
Ferrimagnetic garnet films of, for example, YIG epitaxially deposited on nonmagnetic garnet substrates of, for example, gadolinium gallium garnet (GGG) are known for use in bubble memory devices. These devices are frequently fabricated using the dipping method of LPE. Bubble memory devices usually require a ferrimagnetic film on one face of the substrate only. In order to increase the productivity of dipping method LPE facilities, it has long been the custom to insert two wafer substrates into each set of clamps of a wafer holder for immersion and deposition thereon. A ferrimagnetic film is deposited only on the outer face of each such substrate while the inner faces seal each other from the molten flux.